1. Field
The present disclosure relates generally to interference cancellation in communication systems, and more specifically to apparatus and methods for performing interference cancellation among Quasi-orthogonal Function (QOF) sets in a wireless communication system, such as a cdma2000 system.
2. Background
A wireless multiple-access communication system can concurrently communicate with multiple wireless devices, e.g., cellular phones. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Frequency Division Multiple Access (FDMA) systems.
A wireless multiple-access system typically includes many base stations that provide communication coverage for a large geographic area. Each base station may transmit data to one or more wireless devices located within its coverage area at any given moment. A given wireless device may receive a desired transmission from a serving base station as well as interfering transmissions from nearby base stations. These interfering transmissions are intended for other wireless devices located within the coverage areas of these nearby base stations but act as interference to this given wireless device. The interference hinders the wireless device's ability to demodulate the desired transmission and has a large impact on performance.
Current cdma2000 systems, in particular, are not code limited if using an IS-2000 configuration such as Radio Configuration 4 due to forward link (FL) capacity limitations. With receive diversity or capacity enhancements proposed for the new radio configuration of mobile stations (MS) in cdma2000 1x, the FL capacity can be severely code limited due to a limitation on the number of Walsh codes. It is known, however, to utilize Quasi-Orthogonal Functions (QOFs) as a way to eliminate the code limitations. QOFs consist of the multiplication of Walsh codes by a QOF mask, which is a vector of binary symbols. There are four different orthogonal QOF sets (e.g., sets 1 through 4), with each set being generated using a different QOF mask. The resulting codes are not fully orthogonal to the original Walsh codes, but the masks are configured to minimize the correlation (non-orthogonality) between the generated QOFs and the regular Walsh codes. Users in a QOF set are orthogonal, whereas different QOF sets have minimized cross-correlation.
Linear interference cancellation techniques such as Quasi-linear interference cancellation (QLIC) or Forward Linear interference cancellation (FLIC) may be also utilized to reduce interference from code channels for each sector in a sectortized cell in an active set of a mobile station (i.e., cancel signals from an interfering base station in the mobile station's active set). Examples of the application of QLIC may be found in pending U.S. patent application Ser. Nos. 11/285,510, 11/535,848, and 12/020,417 all of which are assigned to the assignee hereof, and expressly incorporated by reference herein.
Users in other orthogonal sets, which are introduced through the use of QOF (e.g., QOF set 1), are mutually orthogonal within the set, but are not orthogonal to the other orthogonal sets (e.g., QOF set 2). Accordingly, users in one orthogonal set will interfere with users in other orthogonal sets. Thus, in particular technologies, such as cdma2000 1x, there is a need to overcome code capacity limitations through QOF, while reducing or cancelling interference from code channels in a sector through QLIC without interference between users in different orthogonal sets.